Still investigating, but there seem to be multiple problems.

### GPU process memory increases when exiting presentation. 

Issue seems to be a reference loop with XRWebGLDrawingBuffer and its ColorBuffer objects keeping each other alive. A comment in xr_webgl_drawing_buffer.h mentions "DrawingBuffer is explicitly destroyed by the beginDestruction method", but this method wasn't copied over from the original drawing_buffer code. Adding a cleanup function appears to fix this:

void XRWebGLDrawingBuffer::DestroyBuffers() {
  back_color_buffer_ = nullptr;
  front_color_buffer_ = nullptr;
  recycled_color_buffer_queue_.clear();
}

### SharedBuffer mode allocates a Surface but doesn't use it

I have a WIP patch that skips creating the SurfaceTexture/Surface pair in VrShellGl in SharedBuffer mode. With that and the previous patch, memory consumption seems stable.

### Unconfirmed: incomplete Surface cleanup in VrShellGl?

Still investigating, but it appears that VrShellGl may not be fully releasing resources on destruction. I tried adding webvr_surface_texture_->ReleaseBackBuffers() in the destructor, but I'm not sure if there's still an issue with Surface refcounting.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9bc221409571d695d0ee31557351841a7d8a5462

commit 9bc221409571d695d0ee31557351841a7d8a5462
Author: Klaus Weidner <klausw@chromium.org>
Date: Fri Jun 29 18:00:10 2018

Don't create a Surface for WebXR SharedBuffer mode

Managing the Surface/SurfaceTexture pair is somewhat expensive,
and we don't need it for SharedBuffer transport. Now that the
mailbox_to_surface_bridge supports surfaceless initialization
(thanks to AR mode), use that to avoid this allocation.

This required some minor reshuffling to ensure we have the
right initialization order.

BUG=857547

Change-Id: I7edca6e791f9b13bef4a3a50266715182517fdef
Reviewed-on: https://chromium-review.googlesource.com/1120677
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571541}
[modify] https://crrev.com/9bc221409571d695d0ee31557351841a7d8a5462/chrome/browser/android/vr/vr_shell_gl.cc
[modify] https://crrev.com/9bc221409571d695d0ee31557351841a7d8a5462/chrome/browser/android/vr/vr_shell_gl.h

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9fe2370a5c269474a041a316011b41deae036acc

commit 9fe2370a5c269474a041a316011b41deae036acc
Author: Klaus Weidner <klausw@chromium.org>
Date: Fri Jun 29 22:55:08 2018

Avoid WebXR resource leak on destruction

The DrawingBuffer code that WebXRDrawingBuffer is based on requires
a BeginDestruction method to be called to break a reference loop.
Both DrawingBuffer and its ColorBuffer subclass are refcounted and
store references to each other.

Add the BeginDestruction method to WebXRDrawingBuffer which was mentioned
in a copied comment but not implemented.

BUG=857547

Cq-Include-Trybots: luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I3bef8e975c26ca432eafe7eb4ba8c4daae715796
Reviewed-on: https://chromium-review.googlesource.com/1120645
Reviewed-by: Ian Vollick <vollick@chromium.org>
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#571693}
[modify] https://crrev.com/9fe2370a5c269474a041a316011b41deae036acc/third_party/blink/renderer/modules/xr/xr_webgl_layer.cc
[modify] https://crrev.com/9fe2370a5c269474a041a316011b41deae036acc/third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.cc
[modify] https://crrev.com/9fe2370a5c269474a041a316011b41deae036acc/third_party/blink/renderer/platform/graphics/gpu/xr_webgl_drawing_buffer.h